High Tc Superconductors, a complex scenario: defects ordering and nanoscale phase separation

by Alessandro Ricci (Department of Physics, Sapienza University of Rome)

Wednesday, December 7, 2011 from to (Europe/Berlin)
at room 109 ( Bldg. 25b )
Understanding the emerging property of complex matter at nanometer scale phase separation will lead to novel applications and devices.
The scanning X-ray nanodiffraction (nXRD) is a unique new method for high resolution non destructive subsurface interface imaging, with a wavelet mixed real- and reciprocal-space representation of the sample, where the nanostructure associated with short- and medium-range order is captured by means of diffraction and the microstructure is recorded in real space.
Optimally doped La2CuO4+y is ideal for the investigation of intrinsic multiscale heterogeneity in copper oxides because the interstitial oxygens (i-O) are mobile. The La2CuO2+y is an heterostructure with the largest misfit strain in the copper oxides. The ordering of i-O in the La2CuO2+y spacer layers of La2CuO4+y high Tc superconductor (HTcS) is characterized by a scale-free fractal distribution that seem to enhance superconductivity at high temperature [1].
Using X-ray it is also possible to control oxigens ordering and write high-quality superconducting regions probing directly the photo-induced redistribution of mobile i-O. It allows us to correlate nano- and micro-scale structural configurations with material functionality [2].
It is also well known that phase separation is a common feature of HTcS. It has been proposed that the quantum macroscopic phase able to resist to the de-coherence attacks of high temperature takes advantage from material complexity such as frustrated phase separation. nXRD has been used to visualize the frustrated nanoscale phase separation in K0.8Fe1.6Se2 single crystal. It has been possible to map the spatial distribution of two phases : i) the magnetic expanded phase ii) the superconducting compressed phase, pinned to two different Fe-vacancies orders. The results provide a direct evidence for nano phase homogenous domains smaller than 300 nm and different micron size regions with percolating magnetic or superconducting domains forming a multiscale complex network of the two phases [3].
In summary, our works put the HTcS into the same category as other heterogeneous transition-metal, where superconductivity is associated with critical percolation of defects order. Future XPCS studies could be useful to connect the complex granular microstructure, with the defects order dynamic, in HTcS. That is of fundamental importance to understand effects of disorder, photo excitation, pressure and misfit strain in HTcS.